(a) Field of the Invention
The present invention relates to a method for vapor depositing a low dielectric insulating film, and more particularly to a method for vapor depositing a low dielectric insulating film that can significantly increase a vapor deposition speed of a low dielectric insulating film by adding silane gas (SiH4) in a CVD or PECVD process used for forming a protection film of a semiconductor device.
(b) Description of the Related Art
A thin film transistor substrate is used as a circuit substrate for independently operating each pixel in a liquid crystal display, an organic EL (electro luminescence) display, etc. A thin film transistor substrate comprises scanning signal wiring for transmitting scanning signals and picture image signal lines for transmitting picture image signals or data wiring, a thin film transistor connected with gate wiring and the data wiring, pixel electrodes connected with the thin film transistor, a gate insulating film covering the gate wiring to insulate the same, and a protection film covering the data wiring to insulate the same. The thin film transistor comprises a semiconductor layer which forms a channel with a gate electrode that is a part of the gate wiring, source electrodes and drain electrodes that are part of the data wiring, a gate insulating film, and a protection film. A thin film transistor is a switching device which transmits or blocks picture image signals that are transmitted through the data wiring according to scanning signals transmitted through the gate wiring to the pixel electrodes.
The thin film transistor substrate is most commonly used in a liquid crystal display device. As the liquid crystal display device gradually becomes large and fixed, signal-twisting due to an increase in various parasitic capacitances is becoming more problematic. Further, in accordance with the need to improve brightness to increase the visibility range in a liquid crystal display device for a TV and to decrease power consumption in personal computers, an increase in an aperture ratio is needed. In order to increase the aperture ratio, pixel electrodes must be formed overlapping data wiring, but in such a case, the parasitic capacitance between the pixel electrodes and the data wiring increases.
In order to solve the problems caused by an increase in parasitic capacitance, the perpendicular spacing between the pixel electrodes and data lines should be sufficiently secured. In order to secure the perpendicular spacing, the protection film is formed of an organic insulating film. However, a process using an organic insulating film has the following disadvantages. First, the material is expensive. That is, there is a substantial amount of loss during spin coating, which increases material costs. Next, the organic film has minimal heat resistance and thus many limitations are given to subsequent processes. In addition, due to the agglomeration of material, impurity particles are frequently produced, the adhesive strength between an upper film and lower film is low, and etching errors are very large when forming pixel electrodes on a protection film.
Generally, SiO2 and SiN dielectric thin films are used for preparing various shapes of electric devices. There have been ongoing studies of optimized processes for preparing optical wave guides for flat panel displays from silicone IC's.
The material provides the requirements of all devices and strong electrical and mechanical properties. Recently, in order to improve the performance of devices, a conductive layer is more closely contacted or adhered closely together. In addition, in presently used devices, a lower power consumption necessary to allow operation using a battery is considered to be important rather than regular AC coupling. Therefore, in order to decrease electrical capacity coupling causing higher power consumption and slower operation speed, new material having a low dielectric constant is needed.
In order to solve these problems, material based on a Si—C bond of low density exclusive of additional functional groups is used. The material exhibits a low density and thus has a low dielectric constant. Such material is a-SiCOH or silicon oxycarbide, and exhibits a low dielectric constant of 2.7 to 3.5 and thus can greatly improve the performance of a semiconductor device.
Accordingly, if a low dielectric insulating film that is vapor-deposited by a CVD method such as a-SiCOH, a-Si:O:F, etc. is used as an interlayer insulating film and a protection film for a TFT-LCD panel, a coupling capacitance between data wiring and pixel electrodes decreases to lower an RC time delay and cross-talk. Particularly, for a super-high aperture ratio, the pixel electrodes should overlap the data electrodes. However, a SiNx protection film, which is one of the existing protection films, has a very large load capacitance and thus picture images are difficult to realize. Therefore, a dielectric constant of an insulating protection film should be made low as soon as possible, and a sufficient spacing in the perpendicular direction by increasing a thickness of a protection film is necessary. To realize this, a protection film that is vapor-deposited by the CVD method is mainly used.
In order to decrease the coupling capacitance in a-SiCOH film, there is needed a low dielectric interlayer insulating film vapor-deposited by the CVD method, according to its uses, and having a thickness of several micrometers. Since insulating thin films commonly used in semiconductor process are several millimeters in thickness, to compensate a drop in throughput resulting from the significant thickness, a very rapid vapor deposition speed is needed.
a-SiCOH thin film is typically vapor deposited by a PECVD (plasma enhanced chemical vapor deposition) method by adding an oxidant such as N2O or O2 and inert gases such as Ar or He for improving uniformity and stabilizing properties to main source gas SiH(CH3)3 (hereinafter referred to as Z3MS™). For the a-SiCOH thin film, if the flow rate of source gas (for example, trimethylsilane), vapor deposition speed is known to also increase.
However, in the above method, the mechanical strength decreases as the flow rate of the main source gas increases to result in the formation of a thin film that is soft so as to be damaged in subsequent processes. Also, production costs increase because of the increased consumption of the comparatively expensive source gas. Further, if changes are made to various parameters in the CVD vapor deposition process so as to have the lowest dielectric constant (k), the dielectric constant rapidly increases, thereby making it necessary to further increase thickness. As shown in FIG. 1, as flow rate [Z3MS+N2O] of total source gases increases, the dielectric constant linearly increases.